1. Field of the Invention
The present invention relates to a light guide plate of a light emitting device (LED) backlight unit, and, more particularly, to a light guide plate of an LED backlight unit, which has a plurality of elliptic cylinder-shaped grooves carved into an incident plane of the light guide plate in a thickness direction to widen an incidence angle of which light can be transmitted therethrough.
2. Description of the Related Art
A small light crystal display (LCD) for mobile phones and personal digital assistants (PDA) generally employs a lateral light emitting diode (LED) as a light source of a backlight unit. Such a lateral LED is generally provided to the backlight unit as shown in FIG. 1.
Referring to FIG. 1, the backlight unit 10 comprises a planar light guide plate 20 disposed on a substrate 12, and a plurality of lateral LEDs 30 (only one lateral LED is shown in FIG. 1) disposed in an array on a lateral side of the light guide plate 20. Light L entered the light guide plate 20 from the LED 30 is reflected upwardly by a minute reflection pattern 22 and a reflection sheet (not shown) positioned on the bottom of the light guide plate 20, and exits from the light guide plate 20, providing back light to an LCD panel 40 above the light guide plate 20.
Such a backlight unit 20 suffers from a problem as shown in FIG. 2 when light is incident on the light guide plate 20 from the LED 30.
As shown in FIG. 2, light L emitted from each LED 30 is refracted toward the light guide plate 20 by a predetermined angle θ due to difference in refractive index between media according to Snell's Law when the light L enters the light guide plate 20. In other words, even through the light L is emitted at a beam angle of α1 from the LED 30, it is incident on the light guide plate 20 at an incidence angle of α2 less than α1. In FIG. 3, such an incidence profile of light L is shown.
Therefore, there is a problem of increasing a length (l) of a combined region where beams of light L entered the light guide plate 20 from the respective LEDs 30 are combined. In addition, light spots H also called “Hot spots” and dark spots D are alternately formed in the region corresponding to the length (l) on the incident plane of the light guide plate 20. Each of the light spots H is formed at a location facing the LED 30, and each of the dark spots D is formed between the light spots H.
Since the alternately formed light and dark spots are unnecessary for the light guide plate, they must be completely removed, if possible. Even though the light and dark spots are not completely removed, the length (l) must be shortened as much as possible. For this purpose, it is necessary to increase an angle of light entering the light guide plate, that is, an incidence angle of light.
For this purpose, it is suggested to form protrusions on the side surface of the light guide plate as shown in FIG. 4.
Specifically, if a plurality of fine prism-shaped protrusions 24 are formed on a side surface of a light guide plate 20A, light L can enter the light guide plate at an incidence angle α3 substantially equal to an orientation angle α1 of light emitted from a focal point F of a light source. Thus, if orientation angles α1 of light beams emitted from the focal point F of the light source are identical, the light L enters the light guide plate at an incidence angle α3 wider than the case of FIGS. 2 and 3. As a result, it is possible to reduce the problem described in FIG. 2.
Alternatively, a plurality of semi-circular fine protrusion may be formed on the side surface of a light guide plate 20B as shown in FIG. 5. With this configuration, it is possible to obtain the same advantage as that of the configuration shown in FIG. 4.
However, the configurations of FIGS. 4 and 5 suffer from problems as follows.
The light guide plate is generally formed by injection molding of a transparent resin by means of a mold. For example, in the case of forming the light guide plate 20B as shown in FIG. 5, a mold 50 having a shape corresponding to that of the light guide plate 20B carved therein as shown in FIG. 6 is used.
An inner surface of the mold 50 is formed with carved protrusions 52 corresponding to the protrusions 26 of the light guide plate 20B of FIG. 5. Since each of the protrusions 52 has a sharp distal end 54, there is a difficulty in forming the protrusions 52, and even though the protrusions 52 are formed, they can be easily deformed.
Furthermore, in order to allow the mold 50 to be easily separated from the light guide plate after forming the light guide plate by filling the mold 50 with the resin, a release agent is applied to the inner surface of the mold 50. Thus, when the carved protrusions 52 have the sharp distal ends 54, the release agent is often insufficiently or hardly applied to the distal ends 54. As a result, the resin is brought into direct contact with the distal ends 54 of the protrusions 52, so that the light guide plate of the resin is stuck to the protrusions 52 of the mold 50, and is unlikely to be separated from the mold 50 when opening the mold 50 later. Moreover, the light guide plate is likely to be broken at a portion thereof due to an impact upon separation, which increases frequency of defective light guide plates.
Such problems also occur for the light guide plate 20A shown in FIG. 4. That is, since the protrusions 52 are formed on the inner surface of the mold 50 for molding the light guide plates 20A and 20B, the conventional light guide plate causes difficulty in molding operation, and suffers from defective molding.
As an approach to avoid these problems, although it is possible to form the above light guide plates 20A and 20b by cutting away the protrusions 24 or 26 from the side surface of the transparent resin plate, this causes deterioration in operability.
Accordingly, there are needs of a new technique which can overcome the problems related to formability of the light guide plate described above while allowing the fine protrusions to be formed on the side surface of the light guide plate.